This invention is generally related to spraying of articles, and more specifically to an apparatus and method for masking the overspray from a spraying device.
The deposition of metal or ceramic coating to a part using a thermal spraying process is well known. Thermal spraying also known as flame spraying, involves the melting or at least heat softening of a heat fusible material such as a metal, and propelling the softened material in particulate form against a properly prepared surface which is to be coated. The heated particles strike the surface where they quench and bond to the surface. In one type of thermal spray gun, a powder of the coating material is fed axially through a low velocity combustion flame. Alternatively, a thermal spray gun can utilize a high intensity arc to heat inert gas in the gun so as to effect a high velocity gas stream or plasma into which the heat fusible material is injected.
In another type of conventional thermal spray gun, a wire is fed axially through an oxygen-acetylene (or other fuel gas) flame which melts the wire tip. An annular flow of compressed air atomizes the molten wire tip into small droplets or softened particles. The droplets are propelled against a surface by the compressed air. In still another type of traditional thermal spray gun, two wires converge to where an arc between the wire melts the tips to form molten material. The material is atomized and propelled by compressed air against the surface to be coated. All three types of thermal spray are employed to coat various components.
Aluminum alloys are currently being used in automotive components such as internal combustion engine blocks, heads, pistons, bucket tappets and brake rotors to reduce weight and meet governmental fuel economy standards. Other components such as pumps, compressors, transmissions, gear boxes, transfer boxes and axles are also made of aluminum alloys and used in automotive as well as construction, general industry, aerospace and agricultural applications. In addition to aluminum, other materials such as magnesium, zinc, composite metal and polymeric components may be used to reduce cost and improve performance. In most of such applications, there is a need to coat the surfaces of such components in order to withstand thermal-mechanical stresses imposed on them during use.
In one application, such as aluminum engine cylinder blocks, the use of a thermally sprayed coating into the bores of the engine block eliminates the need for inserting cast-iron liners to withstand the sliding contract of steel piston rings or the need to use high silicon content aluminum alloys that require special treatment to precipitate hard wear particles in the bores so as to withstand sliding contact.
When using the thermal spray process, it has been found necessary to mask certain areas of the parts in order to prevent application of the coating in specific adjacent areas. Reasons for masking parts include preventing the coating from entering apertures in the part, maintaining dimensions within a desired range, weight savings and the like.
Three different approaches have been proposed to achieve masking in certain areas. One conventional approach uses a masking tape such as described in U.S. Pat. No. 5,508,097 entitled xe2x80x9cPlasma Spray Masking Tapexe2x80x9d which issued to Hauser et al. on Apr. 16, 1996. Applying a masking tape to surfaces can be time consuming and labor intensive. Thus, the use of a masking tape in high volume thermal spray operations has not met with great success.
Another approach is to control the thermal spray with a spray attenuation member. Examples of the use of such spray attenuation members are shown in U.S. Pat. No. 5,439,714 entitled xe2x80x9cMethod for Thermal Spraying of an Inner Surfacexe2x80x9d which issued to Mori et al. on Aug. 8, 1995 and JP 11106891. However, it is difficult to control overspray at the ends of an inner surface of a part and undesirable non-uniform metal layers can be formed on the inner surface to be coated with this approach.
The third traditional approach is to use masking jigs. Masking jigs are commonly used because they can be positioned by automated equipment to prevent the thermal overspray into specific areas. An external surface masking jig is described in JP 8302459A2. Masking jigs for coating the inside surface of a part such as an engine block, are described in JP 6-287740 and JP 6-65711. Coating the inside surface of a component is more challenging than coating the external surface because of the geometric constraints of accessibility of the thermal spray device and jig into the interior surface area to be coated.
JP 406287740 utilizes a rigid tubular member as a masking jig member. The jig member appears to form a slight gap with the inner diameter of the cylinder bore of an engine block. The masking jig member also appears to move axially in the bore and synchronously with the thermal spray gun as the gun moves in the bore so that substantially all of the overspray is captured in the tubular cavity of the masking member. This unit is complex and requires the tubular jig member to have a slight gap with the surface to be coated to enable the jig to be moved in conjunction with the thermal spray unit. The masking jig must not have a gap that is too large with the inner surface to be coated so as to prevent any substantial overspray past the gap and into masked adjacent areas. However, it may not always be possible to use such a rigid device in cylinder block type applications where the bearing area width-to-bore spacing may limit the size and positioning of such a tubular jig member. Additionally, other geometric constraints at an end of the inner surface of the cylinder bore may prevent forming a slight gap with the inner diameter of the cylinder bore.
Furthermore, JP 406065711A appears to employ a two-part rigid masking jig member with a flange and a tubular portion which can be assembled and disassembled repeatedly for a masking jig. The outside diameter of the assembled masking member appears to have a flat flange that is larger than the inside diameter of the bore and the outside diameter of the cylinder. The masking jig member appears to be assembled within the external end faces of the area adjacent to the crank or bearing journals where the flange is pressed against the bottom end face of the cylinder bore. The thermal spray device is introduced into the bore and the flat flange deflects any overspray back into the cylinder bore. This masking jig most likely has a tendency to form a burr at the interface of the flange and the inner diameter of the bore which is not desirable. Furthermore, the need to assemble and disassemble the masking jig each time the jig is used requires complex and expensive assembly mechanisms.
All of the conventional masking jigs are rigid and non-conformable, and do not permit the use of a rigid masking jig in applications where the distance between bearing caps is less than the diameter of the bore. Thus, there is a need for a conformable jig member that prevents a substantial portion of the overspray from the thermal spray device from deflecting back into the inner surface of the member to be coated and which can deform or conform to fit between bearing cap spaces that are smaller than the bore size of the surface to be coated by the thermal spray.
In accordance with the present invention, the preferred embodiment of a masking apparatus is adapted to mask the overspray of a coating applied by a spraying device. In another aspect of the present invention, a coated article or part includes a member with an inner surface and at least one opening. The inner surface is sprayed with the coating. In yet another aspect of the present invention, the apparatus includes a deformable masking cup which is operably located adjacent to the opening in the article. The masking cup essentially prevents or minimizes overspray from exiting the article past the end of the opening. Another aspect of the present invention provides a method for masking the overspray of a coating.
Thus, the masking apparatus of the present invention is advantageous over conventional devices since the present invention provides a deformable masking cup that is both reusable (or single purpose in an alternate embodiment) to encapsulate the end of the article opening, and is simple and easy to operate. Another advantage of the present invention is that the masking cup is conformable in order to fit between a bearing cap spacing that is less than the bore size of a workpiece such as an engine block. These and other advantages and benefits of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings and claims.